My present invention relates to the determination of the optimum elimination threshold in a radar system for suppressing permanent echoes, applicable inter alia to multi-beam radar systems.
It is known that the detection of targets, mainly low-flying targets, by a radar system is impeded by ambient conditions outside the system, i.e. stationary objects or slow-moving objects or "ground" or "sea" echoes due to the reflection from the soil or from the crests of waves of the energy radiated by the radar system. These conditions make it more difficult for the radar system to detect the required targets, which are surrounded by such undesirable echoes. The echoes, which interfere with proper use of the radar system, are known as "clutter". The term "residual clutter" denotes echoes of this nature surpassing a pre-established threshold level, resulting in so called false alarms.
Some known devices are used in an attempt to obviate the aforementioned disadvantages. These devices, which eliminate permanent echoes, are known as "moving-target indicators" (MTI).
One of the conventional devices of this type, called an "Area MTI", eliminates permanent echoes in one area at a time and will be further described here since the system according to the invention is based on the same principle. A discussion of an Area MTI can be found in Barton (Prentice Hall Series 1964) Radar System Analysis, page 224, or in Merrill Skolnik (Mc Graw Hill Book Co 1970) Radar Handbook, pages 17-54 and 17-55.
The radar area to be processed is divided into elementary compartments which receive a certain video-frequency energy at each revolution of the antenna. Upon each revolution, this energy is compared with a threshold which has been set to allow for the average energy received from a surveyed zone during the preceding antenna revolutions. The threshold value S.sub.c (i- 1), set at the (i- 1)th antenna revolution, and compared with the energy received at the i.sup.th revolution is automatically adjusted for each zone thus swept by the antenna beam.
FIG. 1 shows the conventional method of setting the threshold used to determine the presence or absence of an echo from the target. The variation in the threshold S.sub.c (i) is given in dependence on the antenna revolutions i, and the drawing shows the level CL corresponding to the noise amplitude in the swept zone and the energy corresponding to the undesired signals or clutter. Under these conditions, the desired signal corresponds to the instantaneous increase in average energy when a moving target travels through the compartment.
Accordingly, the threshold value is increased by a value M whenever an input signal occurring in the swept zone has an amplitude greater than the previously-set threshold, in which case an echo indication is transmitted to the radar receiver for display in the usual manner.
On the other hand, the threshold value is reduced by a value N if no input signal is present or if a signal is present and its amplitude is less than the previously-set threshold.
The values of M and N used to update the basic threshold S.sub.c are integers and different from one another, the increment M being made greater than the decrement N.
Accordingly, if Vi is the amplitude of the signal received by the compartment in question at the i.sup.th antenna revolution, S.sub.c (i-1) is the threshold set before the last-mentioned antenna revolution but used as a comparison value for the energy received at the following or i.sup.th revolution, and S.sub.c (i) is the threshold value obtained and therefore calculated after comparing the voltage Vi with the threshold S.sub.c (i-1), then, if Vi &gt;S.sub.c (i-1), i.e. if there is an echo, the threshold value becomes S.sub.c (i) = S.sub.c (i-1) + M for the following or (i+1).sup.th antenna revolution.
If Vi &lt; S.sub.c (i-1), there is no echo and the threshold is set at S.sub.c (i) = S.sub.c (i-1) - N.
Consequently, assuming that the amplitude of the return or echo signal remains constant, the variable threshold S.sub.c (i) is held close to the clutter level CL. The residual clutter rate tx can be shown to be: ##EQU1##
It can be seen that the residual clutter rate is important, since it can be used for more accurately determining the modifications to be made to the set threshold in order to ensure optimum elimination of permanent echoes under various practical conditions.
According to (FIG. 1) the threshold value automatically follows slow fluctuations in the amplitude of the received signal, since the value of the threshold is periodically set at less than the signal amplitude CL due to permanent echoes. In accordance with formula (1), this comparison gives rise to a non-zero residual rate.
The object of the invention is to obtain a low residual clutter rate so as to improve the elimination of permanent echoes.
I realize this object, in accordance with the present invention, by comparing an input signal Vi from a recurrently swept zone with a variable threshold S.sub.c, conventionally established as explained above, while modifying that threshold by the addition (or subtraction) of a supplemental parameter K whose magnitude, in turn, is varied in dependence upon deviations of the residual clutter rate from a standard. More specifically, as long as the ratio of the number of echoes surpassing the variable threshold to the number of echoes surpassing a predetermined fixed threshold Sf stays within given limits, parameter K is not changed; otherwise, its value is altered in a sense tending to eliminate the aforementioned deviations, i.e. to restore the residual clutter rate to a range of numerical values defined by these limits. Thus, the modified threshold represented by the algebraic sum of basic threshold S.sub.c and supplemental parameter K is increased if the aforementioned ratio exceeds the upper range limit (positive deviation) but is reduced if it falls short of the lower range limit (negative deviation).
Advantageously, any change of the supplemental parameter K during a sweep of a surveyed zone is utilized in the next-following sweep of the same zone; thus, an echo is registered on the radar display whenever the input signal Vi in the current sweep i exceeds the algebraic sum of S.sub.c (i-1) and K(i-1).
In this way the display rate of permanent echoes or false alarms is stabilized at an optimum value. The described technique can be extended to a multi-beam radar system, either by applying the given result to each beam or by first defining a threshold from a mixture of video signals from all the beams in question, or from groups of beams, and suubsequently modifying the threshold for each beam or group of beams.